The present invention relates generally to manufacturing test equipment, and more particularly, to laser rangefinder test equipment that provides for laser range simulation.
In the early 1970's, the assignee of the present invention developed a range simulator known as a simulated optical range target (SORT). This range simulator utilized a long aluminum clad optical fiber, a termination mirror, and a thick lens collimator to simulate a target at a given distance. In operation, the SORT is placed in front of a laser rangefinder under test, the center of the laser rangefinder's reticule is superimposed on the SORT's optical fiber and the laser rangefinder is fired. The output pulse of laser light from the laser rangefinder is attenuated and focused by the thick lens collimator onto the single long optical fiber. The light travels down the fiber until it reaches the termination mirror, i.e., the simulated target, placed at the end of the fiber. Upon striking the termination mirror, the laser light is reflected back up the fiber, back through the thick lens collimator, and into the laser rangefinder. The rangefinder then calculates the range of the simulated target so that an operator may compare the calculated range to the calibrated range for the particular SORT being used. A laser rangefinder determines target range by calculating the time needed for the laser light emitted by the laser rangefinder's laser to travel to a target and return to the laser rangefinder. Although the simulated optical range target is still in limited use, it suffers from several major drawbacks.
Because the glass optical fiber used in the SORT does not transmit light at mid and far infrared wavelengths, i.e., wavelengths beyond 2,000 nanometers (nm), the simulated optical range target cannot be used to test laser rangefinders which operate at wavelengths beyond 2,000 nm. A common example of a laser rangefinder which operates in the far infrared region of the electromagnetic spectrum is the CO.sub.2 laser rangefinder which operates at 10,060 nm. Optical fibers which transmit light in the infrared wavelengths currently do not exist in a form that could be adapted for use with the SORT. Another limitation is the physical length of the optical fiber cannot be increased once a SORT is manufactured, the simulated target range is therefore not variable but limited to that range established at the time the particular SORT was constructed.
Other limitations with SORT revolve around its calibration. A laser target range facility is needed to calibrate the simulated optical range target. This facility, usually constructed outdoors, typically requires at least one laser target placed at a visually unobstructed minimum surveyed distance in excess of 200 meters (typically 500 meters) from the operational laser rangefinder which will be used in the SORT calibration. The laser rangefinder is first fired at the real target and its calculated range compared to the surveyed range of the test target. This verifies the proper operation of the laser rangefinder. The rangefinder is then used to calibrated the SORT. Since real laser target range facilities are extremely costly to construct and maintain and since the propagation of laser radiation through the atmosphere is susceptible to adverse weather conditions, the calibrated value, of a particular SORT, is typically verified only once (at the time of manufacture) and is only as good as visibility and other weather conditions on the day of calibration. Atmospheric conditions, such as (light) scattering, (light) transmission, etc., vary daily (or even hourly), an absolute, repeatable, calibration of a particular SORT can therefore rarely be accomplished.
A final major problem with the simulated optical range target is its difficulty in accurately simulating multiple target returns. Most factory acceptance tests require that a laser rangefinder demonstrate its ability to distinguish between two targets placed in close proximity (typically 10-20 meters apart) to each other. Although adequate for in field testing of laser rangefinders, the lack of a reliable absolute calibration and the difficulty in simulating multiple targets has rendered the use of SORT unacceptable for factory acceptance testing.
In the late 1970's, the assignee of the present invention worked on an electronic simulated range device (SRD). This device utilized a laser diode to opto-electrically simulate the return signal that a laser rangefinder would see when fired at a target. In operation, the SRD was placed in front of the laser rangefinder to be tested and the laser rangefinder was connected to the simulated range device through a test connector. The test connector allowed access to the laser rangefinder's "A-trigger" signal. This is the signal generated when the laser rangefinder's laser is fired; it is the signal that starts the laser rangefinders range counter circuit. A return signal delay time was set into the SRD's electronics prior to testing the laser rangefinder. This delay time would correspond to the time it would normally take for the laser rangefinder's emitted laser light to travel a particular distance (target range) and return to the laser rangefinder under test.
An operator would then fire the laser rangefinder to begin the test. Upon firing, the laser rangefinder's "A-trigger" started the simulated range device's counter and at the selected delay time, the simulated range device fired its laser diode into the laser rangefinder's receiver to simulate the target's return signal. The operator could then compare the laser rangefinder's calculated range to the range for the specific delay time set on the simulated range device. For convenience, the SRD's delay time was displayed on the SRD's control panel as a range. Although the simulated range device allowed limited testing with laser rangefinders that utilize non-common transmitter and receiver optics, SRD development was suspended after several years because of its inability to successfully test laser rangefinders that utilize common optical paths. The simulated range device suffered from several shortcomings.
The simulated range device would not work with laser rangefinders that utilize common optical paths because with common optical paths, the SRD's diode laser was destroyed by its exposure to the laser radiation generated by the laser rangefinder's output beam when the laser rangefinder under test was fired. The SRD only operated at two wavelength (694.3 nm and 1060 nm) and could therefore only be used to test laser rangefinder's operating at these wavelengths. Since the output energy of the laser rangefinder under test was not measured, no attempt was made to set the SRD laser diode's pulse to an output energy level that would represent the amount of energy that the laser rangefinder would see reflected from a real target. No attempt was made to set the optical characteristics (polarization, pulse width, etc.) of the SRD's diode laser's output pulse to represent the optical characteristics of that the laser rangefinder would see reflected from a real target.
Accordingly, it is an objective of the present invention to provide for a target range/extinction simulator that can be electronically calibrated and eliminates the need for an outdoor laser test facility. It is another objective to allow a laser rangefinder that utilizes a common transmitter and receiver optical path to be tested against both single and multiple simulated targets. It is a further objective to provide for manufacturing test equipment that permits increased production flow (thus reducing cost) by eliminating the necessity to move laser rangefinders to and from a laser test facility and to further provide for field test equipment that permits testing of laser rangefinders in areas where laser operation would otherwise be prohibited because of eye injury hazards.